A laser machining apparatus is configured to perform machining such as piercing or cutting by radiating a laser beam, which is condensed to have higher power density, to a machining object such as a metal material and a resin material. A lens for condensing the laser beam is called a machining lens, which absorbs a part of the laser beam transmitting therethrough and increases in temperature. The heat diffuses from a central part of the lens through which the laser beam transmits toward an outer peripheral part of the lens. Thus, there occurs a temperature distribution in which a temperature is higher at the central part of the machining lens and is relatively lower at the outer peripheral part thereof.
Meanwhile, a refractive index of a material for the machining lens has temperature dependence. Thus, when the machining lens has the temperature distribution, the temperature distribution causes a distribution of the refractive index. As a result, what is called a thermal lens effect is generated.
The refractive index distribution varies along with time of the transmission of the laser beam, and is converged to a steady state value with a predetermined time constant. In other words, a magnitude of the thermal lens effect tends to vary along with the elapse of a machining time, and then tends to be saturated to a predetermined magnitude.
When the laser beam is condensed with the machining lens and radiated to the machining object, the thermal lens effect causes a variation in focal length of the machining lens and a variation in beam diameter of the laser beam radiated to the machining object. Further, the magnitude of the thermal lens effect varies along with the elapse of the machining time, and the beam diameter varies as well along with the machining time. As a result, machining becomes unstable, which may cause machining defects.
In view of the circumstance, in order to prevent the variation of the beam diameter on the machining object, which is caused by the thermal lens effect, and the temporal variation of the beam diameter during the machining step, there has been proposed a laser machining apparatus having a focal position automatic correction function of detecting, with a far-infrared radiation thermometer and a thermocouple, temperatures of a laser irradiation part on the machining lens and a peripheral part of the machining lens, and correcting a clearance between the machining lens and the machining object based on the temperatures thus detected so as to offset the thermal lens effect, to thereby stabilize the beam diameter of the laser beam on the machining object (refer, for example, to Patent Literature 1).
In the following, description is made of a conventional laser machining apparatus including a focal length automatic adjustment device with reference to FIG. 10. FIG. 10 is a view of a structure of the conventional laser machining apparatus described in Patent Literature 1.
In FIG. 10, a temperature of a central part of a machining lens 31 at the time of entry of a laser beam 32 is measured with a far-infrared radiation thermometer 34 set at a position distanced from the machining lens 31, and a temperature of a side surface of the machining lens 31 is measured with a thermocouple 33. Measurement results of those temperatures are input to a microcomputer 36, and a required lens moving amount is calculated. In this way, a position of the machining lens 31 is adjusted with a Z-axis stage 38 in directions of an optical axis of the laser beam 32.
The machining lens 31 absorbs a part of the laser beam 32 at the time of transmission of the laser beam 32 therethrough, and heat absorbed simultaneously therewith flows toward an outer periphery of the machining lens 31. As a result, the temperature of the central part of the machining lens 31 increases, and the temperature of the outer peripheral portion decreases in contrast. In other words, a radial temperature distribution occurs in the machining lens 31, and a phenomenon called a thermal lens is generated at this time.
A refractive index of a material for the machining lens 31 has temperature dependence, and hence when the temperature distribution occurs, a refractive index distribution occurs as well. In other words, the thermal lens refers to a lens effect caused by the refractive index distribution. However, it is necessary to notice that the temperature itself of the machining lens 31 does not cause the thermal lens. The thermal lens of the machining lens 31 is normally a convex lens component. When the thermal lens occurs, a focal length of the machining lens 31 varies, with the result that a diameter of the beam radiated to the machining object varies.
Further, after the start of irradiation of the laser beam 32 to the machining lens 31 at the start of machining, the temperature distribution of the machining lens 31 approaches a steady state value with a certain time constant. Thus, the magnitude of the thermal lens varies during the machining. In other words, the diameter of the beam radiated to the machining object varies during the machining. As a result, the machining may become unstable, or machining defects may occur.
In order to prevent this, it may be appropriate to change the position of the machining lens 31 in accordance with the magnitude of the thermal lens so as to correct the variation of the focal length. Note that, as describe above, the magnitude of the thermal lens varies during the machining, and hence the magnitude of the thermal lens needs to be detected in real time during the machining.
The thermal lens is influenced by the temperature distribution, and hence the magnitude of the thermal lens can be grasped through measurement of the temperature distribution. In Patent Literature 1, the thermal lens is calculated through measurement of the temperatures of the laser beam irradiation portion on the machining lens 31 and the peripheral portion of the machining lens 31 with the thermocouple 33 and the far-infrared radiation thermometer 34.